Abstract Ridge ice condition is a critical state of aircraft icing, while the unsteadiness of flow oscillation to post-stall leading is concerned by researchers. This paper uses Shanghai Jiao Tong University Superman CFD code to resolute the unsteady flow behind a NACA 23012 airfoil with three-dimensional spanwise ice. Results have shown that the shear layer from the ridge has not reattached the wall, while the other from trailing edge produced “up-wash” flow. Two large-scale coherent structures have intensive interactions at the wake. The dominant (1st and 2nd) mode is extracted by proper orthogonal decomposition method. The phase of POD domainant mode is in common with the instability of interaction between the large-scale coherent structures. By comparing the similar frequency with that of instantaneous lift coefficient CLdocumentclass12pt{minimal} usepackage{amsmath} usepackage{wasysym} usepackage{amsfonts} usepackage{amssymb} usepackage{amsbsy} usepackage{mathrsfs} usepackage{upgreek} setlength{oddsidemargin}{-69pt} begin{document}$$C_L$$end{document}, the decomposed mode implies that the main cause of lift fluctuation near stall comes from the instability at the wake rather than a single shear layer.
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